There have been reported a number of processes for producing 3-oxocarboxylic acid esters which are called .beta.-ketoesters in general.
For example, Chem. Rev., 95, 1065-1114 (1995) discloses a process for producing .beta.-ketoesters which comprises condensing a carboxylic acid chloride with a malonic acid ester or an acetoacetic ester in the presence of a base followed by decarboxylation or deacetylation. As the base component to be employed in this process, use is made of, for example, sodium hydride, sodium amide or alkali metal alcoholates.
As improvements in the base component in the reaction for condensing an acetoacetic ester with a carboxylic acid chloride, further, it is suggested to use magnesium alcoholates Helv. Chem. Acta., 35, 2280 (1952)! or calcium compounds (U.S. Pat. No. 5,194,671).
Furthermore, U.S. Pat. No. 5,144,057 and JP-A-57-70837 each discloses a process for producing .beta.-ketoesters which comprises reacting a magnesium complex or a calcium complex of an acetoacetic ester with a carboxylic acid chloride (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
However these processes with the use of sodium hydride, sodium amide, alkali metal alcoholates, etc. as the base component suffer from a problem that the .beta.-ketoesters thus formed are liable to further undergo the secondary reaction and thus only a poor yield can be achieved.
When a magnesium alcoholate is employed as the improved base component, on the other hand, there arises a problem in safety, i.e., the evolution of explosive hydrogen gas in the process of the formation of the alcoholate from metallic magnesium and an alcohol.
When the above-mentioned process with the use of a calcium compound is applied to the production of 3-oxocarboxylic acid esters, only a poor yield can be achieved for the following reasons 1) and 2). In addition, this process brings about another problem of environmental pollution.
1) Aqueous ammonia, which is employed to promote the deacetylation of the intermediate formed from the 3-oxocarboxylic acid ester and acetoacetic ester, causes the formation of carboxylic acid amide crystals as the by-product. Thus the yield is lowered and the post-treatment becomes troublesome.
2) To activate the calcium compound, it is needed to use methylene chloride, i.e., a halogen compound which is likely to cause environmental pollution.
Further, the above-mentioned process with the use of magnesium enolate of an acetoacetic ester involves a troublesome procedure of adding a tertiary amine in the step of the acylation through the reaction with a carboxylic acid chloride. In the above-mentioned process with the use of a magnesium complex or a calcium complex, furthermore, the acylation reaction should be performed at a high temperature.
For these reasons, it has been urgently required to develop a process for producing .beta.-oxocarboxylic acid esters which is economically advantageous and applicable to industrial production.
The present inventors have conducted extensive studies on a process for producing 3-oxocarboxylic acid esters which are useful as intermediates in the synthesis of ceramides to be used as a humectant, biodegradable polymers, drugs, etc. so as to solve the problems described above. As a result, they have successfully established a process which is economically advantageous and applicable to industrial production, thus completing the present invention.